In microwave-assisted chemistry, microwaves are used to initiate, drive, or otherwise enhance chemical or physical reactions. Generally, the term “microwaves” refers to electromagnetic radiation having a frequency within a range of about 108 Hz to 1012 Hz. These frequencies correspond to wavelengths between about 300 cm to 0.3 mm. Microwave-assisted chemistry is currently employed in a variety of chemical processes. Typical applications in the field of analytical chemistry include ashing, digestion and extraction methods. In the field of chemical synthesis, microwave radiation is typically employed for heating reaction materials, many chemical reactions proceeding advantageously at higher temperatures. In addition, when pressureriseable reaction vessels are used, many analytical or synthetical processes can be further enhanced by increasing the pressure in the vessel. Further, when, for example, digestion methods for analytical purposes are used, the generation or expansion of gases inside the vessel will necessarily increase the internal pressure. Thus, in order to ensure that no reaction products are lost for subsequent analysis, vessels must be used which are able to withstand high internal pressures in these cases.
Usually, most microwave-assisted reactions are performed in open or, preferably, in sealed vessels at temperatures rising up to 300° C. Typical pressures range from below atmospheric pressure, e.g. in solvent extraction processes, up to 100 bar, e.g. in digestion or synthesis processes.
Microwave-assisted chemistry is essentially based on the dielectric heating of substances capable of absorbing microwave radiation, which is subsequently converted into heat.
Many apparatuses and methods currently employed in microwave-assisted chemistry are based upon conventional domestic microwave ovens operating at a frequency of 2.45 GHz. As magnetrons operating at this frequency are produced in large quantities for domestic appliances, microwave apparatuses for microwave-assisted chemistry using such magnetrons can be manufactured at relatively low cost.
The microwaves generated by the magnetron are coupled via an antenna into a waveguide and transferred into a resonance cavity of the microwave oven. In order to avoid that microwave energy is reflected back into the waveguide, which might then damage the magnetron, care has to be taken to match the impedance of the waveguide and the impedance of the oven where the sample is arranged and to ensure that a sufficient amount of microwave energy is absorbed in the resonance cavity. Using appliances having a form factor of conventional domestic microwave appliances requires both that samples having a high absorbance for microwave radiation are employed and that relatively large amounts of these samples are present in the oven.
However, when larger amounts of samples are heated with microwave radiation, the problem arises that the depth of penetration of microwaves into the sample is relatively small. Consequently, direct microwave heating will only occur in sample areas which are close to the surface of the sample and the bulk of the sample will only be heated via thermal conductivity or, if liquid or gaseous samples are heated, by thermal convection. Specifically, insufficient heat transfer will often lead to inhomogeneous heating of the sample. Stirring of the sample only partly mitigates this problem because the larger the sample volume, the more difficult it is to reliably control the stirring and heating process.
In many applications, such as analytical chemistry and chemical synthesis, uniform heating of the samples is of utmost importance since, for example, reaction rates strongly depend on the temperature of the sample.
As already noted above, apparatuses using resonance cavities require a delicate impedance matching between the waveguide which transmits the microwave radiation from a microwave source to the resonance cavity and the resonance cavity itself. The resonance conditions within the cavity are, however, dependent on the samples to be heated, the type and filling level of solvents or reactants employed, etc. In addition, during the course of the chemical or physical process induced by microwave radiation, drastic variations of the dielectric properties of the samples, solvents or reactants may occur. In summary, in these prior art methods it is rather difficult to ensure that an effective, uniform and reproducible heating of samples, in particular larger amounts of samples, is achieved.
Various approaches which do not employ resonance cavities and which try to overcome the limitations associated with the use of rather conventional microwave ovens have been described in the prior art.
In GB 2 206 470, a cooker appliance is described in which radio frequency radiation having a frequency of typically 14 MHz is applied to food to be heated by means of a coaxial slow-wave transmission line arrangement. The use of comparatively low frequencies as compared to microwave radiation ensures a larger penetration depth of the radiation. While able to heat food, this appliance hardly produces enough radio frequency energy to heat larger samples to the temperatures required in microwave-assisted chemistry.
U.S. Pat. No. 6,294,772 describes a microwave probe applicator for physical and chemical processes which can be arranged within the reaction vessel, i.e. directly within the sample to be heated. This type of applicator produces a rather inhomogeneous microwave field within the sample.
In U.S. Pat. No. 3,848,106, an apparatus for heating by microwave energy is described which employs a dielectric material having a constant cross-sectional dimension in the direction of propagation of the microwave energy, which is arranged close to the sample to be heated and which enables to heat samples arranged close to the surface of the dielectric material. Due to the decreasing field strength in the direction of propagation of microwave radiation, the heating of the sample will not be homogeneous either.
WO 2005/043953 describes a continuous feed microwave applicator for heating food which is fed via a conveyor belt into a tapering application. This device is not suitable for heating liquid samples.
U.S. Pat. No. 4,067,683 describes a method and apparatus for controlling the fluency of hydrocarbon fluids by directing electromagnetic radiation through a dielectric cone into the fluid. While the apparatus of U.S. Pat. No. 4,067,683 might be sufficient to control the fluency of hydrocarbon fluids, no homogenous heating of the sample is possible.
WO 90/0910 describes a fluid pumping apparatus comprising a pipe section having a microwave transparent window allowing microwave energy to be directed into the pipe section to elevate the temperature of the fluid within the pipe section. This device does not allow homogenous heating of the fluid within the pipe either.
U.S. Pat. No. 3,555,232 describes a rectangular waveguide for heating samples within a central area of the waveguide. Longitudinally tapering ridges are provided within this waveguide in order to vary the ration of intensification to rarefaction of the electromagnetic filed. The provision of ridges renders the internal structure of the waveguide rather complex.
U.S. Pat. No. 3,474,209 discloses a method and apparatus for heat treatment of an article in a hollow waveguide having a non-linear taper in one dimension thereof. With and height of the waveguide have to be adapted to allow propagation of TE01 waves only. Consequently only relatively flat articles positioned in the centre of the wave guide can be heated uniformly.